Hmm, sounds like the elephant smoked too much if the keys have turned yellow.  (Sorry, but I couldn't help passing on an old joke originated by pianist Victor Borge.)

First, you must ascertain that the key tops are actually made of ivory, instead of a plastic coating.  True ivory keys were present on pianos made before about 1950.  The coverings consist of two pieces of ivory per note:  One on the wider "natural" part of the note, and the second half is in the narrow strip between the black notes.  If you do not see a horizontal seam between these two sections of the keys (i.e., if they are "one-piece ivory") then they are actually a form of plastic and not ivory.

A Google search in the technician's section of pianoworld will reveal that a very fine abrasive, such as toothpaste, will clean ivory keys.  Under no circumstances, use any type of bleach to whiten natural ivory keys.  Don't count on the sun's UV whitening the keys.  Why not? If the sunlight hasn't lightened the keys in the past 50+ (more like 70+ years), then it won't help in the future.

Cheers,
Joe

P.S.  Rather than relying on any number of opinions on the internet, it is probably best to telephone the advice of an experienced Piano Guild technician.

Hahahah that is a very good joke lol. I will have to tell it to my friend.
Funny thing is that I did not see that horizontal line you speak of. I will have to inspect it further tommorow.

I will also talk to some piano technician because that is probably the best. Thank you for sharing the toothpaste thingy because I didnt find that when researching

First, you must ascertain that the key tops are actually made of ivory, instead of a plastic coating.  True ivory keys were present on pianos made before about 1950.  The coverings consist of two pieces of ivory per note:  One on the wider "natural" part of the note, and the second half is in the narrow strip between the black notes.  If you do not see a horizontal seam between these two sections of the keys (i.e., if they are "one-piece ivory") then they are actually a form of plastic and not ivory.

A Google search in the technician's section of pianoworld will reveal that a very fine abrasive, such as toothpaste, will clean ivory keys.  Under no circumstances, use any type of bleach to whiten natural ivory keys.  Don't count on the sun's UV whitening the keys.  Why not? If the sunlight hasn't lightened the keys in the past 50+ (more like 70+ years), then it won't help in the future.

Cheers,

Joe

P.S.  Rather than relying on any number of opinions on the internet, it is probably best to telephone the advice of an experienced Piano Guild technician.

I was searching for the ideas on this topic and have found various conflicting opinions..
Here it says that you can use the white vinyl eraser to whiten the keys but I have also heard that that is not good because it can leave marks on the keys..

Also I have read that you can expose the keys to the sunlight and that they will naturally brighten from it?
But what if the direct sunlight damages them? The keys in question were in the basement for a long long time and didn't see the light of day ever during that time. So now I am afraid that the UV radiation can actually degrade them.

Overall the keys are in good condition but they have that yellowish patina formed on them because they were neglected for a long time. They weren't even cleaned while the piano was in use like 50 years ago.

What are your thoughts on this? Do you have any good idea how to do this?